Copper enzymes exhibit a range of reactivities with dioxygen. Small molecule models offer a means of probing the fundamental problems of this Cu/O2 chemistry, in a detailed and systematic way. Several small molecules, relevant to the active site chemistries of O2-reactive mono-, bi-, and trinuclear copper enzymes, (e.g., galactose oxidase, hemocyanin/tyrosinase, laccase, particulate methane monooxygenase) have been prepared. These molecules share the common theme of addressing reactive, highly-oxidized species produced in the process of Cu/O2 or Cu/oxidant reaction (e.g., antiferromagnetically coupled Cu(II)-tyrosyl or Cu(III) systems). In particular, a small-molecule-based, XAS investigation of Cu(III), of Cu(II) coordinated to ligand radicals, and of the chemical factors which stabilize one or the other of these reactive configurations, is proposed.